The present invention is directed generally to amplifiers, and more specifically to an output buffer which is electrically alterable to be permanently converted between push-pull configuration and open-collector configuration or vice-versa.
The prior art has included convertible amplifiers which generally have mechanical switches such as manually operated switches or relays or transistor switches to select a desired configuration. These circuits have not been used in integrated circuits.
Integrated circuits having transistor-to-transistor logic (T.T.L.) output buffers are commonly configured as open-collector or push-pull. The standard approach taken by semiconductor manufacturers is to include within the circuitry of the buffers the components necessary to provide the push-pull configuration and by means of a metal option create the open-collector configuration. Two metal masks are required to provide the simple option. Also, two sets of test programs are required at the final test area for all D.C. tests. All tests must separate open-collector from push-pull configurations due to inevitable product mix at both the wafer, assembly and final test rescreen levels. This leads to serious logistics problems of maintaining product identity in the wafer fabrication process as well as in the final test areas. Thus there exists a need for a methodology of configuring amplifiers or output buffers in a single configuration which may be electrically altered after final testing to allow selection of a desired permanent configuration.